1. Field of the Invention
The present invention relates to a fabrication process of dynamic random access memory (DRAM), and in particular to a method for forming a bottle-shaped trench.
2. Description of the Related Art
Generally speaking, the capacitors widely used in dynamic random access memory (DRAM) are formed by two conductive layers (electrode plate) having an insulation layer in between. The ability to store electric charges of a capacitor depends on the thickness of the insulation layer, surface area of the electrode plate and the electrical characteristics of the insulation material. In the recent development for the design of process for reducing sizes of semiconductor elements to enhance integration density, the area of memory cells in a memory must continuously be reduced to support a larger number of memory cells, thereby increasing density. Meanwhile, the electrode plates of a capacitor in a memory cell must contain sufficient surface area to store sufficient electrical charge.
Nevertheless, with sizes of elements continuously reduced, trench storage node capacitance of DRAM is decreased as well. As a result, storage capacitance must be increased to maintain good operating performance for memory.
Currently, the method for increasing storage capacitance for DRAMs increases the width of the bottom of the trench, thereby increasing surface area to form a bottle-shaped capacitor. The above method is carried out by selective oxidation of the upper portion of a trench to form a collar oxide layer to protect the upper portion of the trench. Next, the lower portion of the trench is wet-etched to form a bottle-shaped trench having a greater radius than the upper portion of the trench.
In a conventional process, a trench is formed by isotropic dry etching on a semiconductor substrate having a pad stack layer comprised of an oxide layer and a nitride layer formed thereon. Next, a nitride layer, an oxide layer, polysilicon layer and another oxide layer are sequentially formed on the pad stack layer and the trench. Nevertheless, the multiple deposition steps further increase the complexity of the process. Therefore, the traditional method is not economical due to high production costs and longer process time. Hence, a simplified process with high production yield for forming bottle-shaped trenches is required.
In order to solve the conventional problems, the invention utilizes only two depositions of oxide layers, followed by forming a sidewall protection layer in the upper portion of the trench. A bottle-shaped trench is then formed by wet stripping to increase the radius of the lower portion of the trench.
Accordingly, an object of the invention is to provide a method for forming bottle-shaped trenches that is simplified and economical.
The method for forming bottle-shaped trenches provided in the invention, including providing a semiconductor substrate having a pad stack layer thereon and a trench in a predetermined position, forming a first dielectric layer conformally on the pad stack layer and the sidewalls of the trench, filling the lower portion of the trench with a sacrificial layer, removing the first dielectric layer from the pad stack layer and the upper sidewalls of the trench, removing the sacrificial layer and leaving the first dielectric layer located on the lower sidewalls of the trench, forming a second dielectric layer conformally on the pad stack layer and the upper sidewalls of the trench and filling the lower portion of the trench to fully close the opening of the first dielectric layer, forming a protection layer conformally on the second dielectric layer, recessing the protection layer so that only the protection layer located on the upper sidewalls of the trench remains to form a sidewall protection layer, removing the second dielectric layer located at the bottom of the trench to form an opening, using the sidewall protection layer as a mask to remove the first dielectric layer and the second dielectric layer located at the lower portion of the trench to expose the lower sidewalls of the trench, and removing the lower sidewalls of the trench to form a bottle-shaped trench by wet stripping.
The pad stack layer, composed of a nitride layer and an oxide layer, is used as a hard mask for subsequent etching. The first dielectric layer and the second dielectric layer are dielectric material, such as tetraethylorthosilane (TEOS), oxide by SACVD, or oxide by plasma enhanced CVD. Formation of the dielectric layers is preferably low pressure CVD. High density plasma (HDP) is also applicable for the second dielectric layer. Sacrificial layer is preferably photoresist material, and the protection layer is preferably dielectric such as nitride.
Furthermore, removal of dielectric layer is carried is out by conventional methods, such as wet etching using diluted HF (DHF) or buffered HF (BHF).
According to the method for forming bottle-shaped trenches provided by the invention, process is greatly simplified and production time and costs are reduced. Thus, the method is more economical than conventional methods.